1. Field of the Invention
The present invention relates to a temperature sensor, a sulfur component detector, and an exhaust purification system for an internal combustion engine.
2. Description of the Related Art
In an exhaust purification system for an internal combustion engine that includes a NOx storage-reduction catalyst, a sulfur component in exhaust gas is stored in the NOx storage-reduction catalyst, and thereby decreases NOx purification efficiency of the NOx storage-reduction catalyst.
When the NOx purification efficiency of the NOx storage-reduction catalyst is decreased by sulfur poisoning, the NOx storage-reduction catalyst is heated in a rich air-fuel ratio atmosphere to regenerate the catalyst. However, this results in thermal aging of the NOx storage-reduction catalyst, which is caused by the catalytic regeneration, and reduced fuel efficiency.
Considering the above, as shown in FIG. 4, a sulfur trap device 20 is disposed upstream of the NOx storage-reduction catalyst 30 and traps sulfur in the exhaust gas at a location upstream of the NOx storage-reduction catalyst 30 in order to prevent sulfur poisoning of the NOx storage-reduction catalyst 30. Accordingly, the thermal aging of the NOx storage-reduction catalyst 30, which is caused by the catalyst regeneration, and deterioration in fuel efficiency are prevented. The arrow A in FIG. 4 indicates a flow direction of the exhaust gas.
However, when the amount of sulfur accumulated in a sulfur trap catalyst or a sulfur adsorbent exceeds a threshold capacity in the sulfur trap device 20, the sulfur component passes through the sulfur trap device 20. The NOx storage-reduction catalyst 30 downstream of the sulfur trap device 20, is then subjected to sulfur poisoning. Accordingly, the sulfur trap catalyst or the sulfur adsorbent needs to be replaced. More specifically, it is a precondition of the configuration in FIG. 4 that the NOx storage-reduction catalyst 30 does not include catalytic regeneration means. Therefore, before the NOx storage-reduction catalyst 30 becomes subjected to sulfur poisoning, the decreased efficiency of the sulfur trap device 20 has to be estimated, and accordingly, the sulfur trap catalyst or the sulfur adsorbent has, to be replaced. In the related art, an amount of the sulfur component that passes through the sulfur trap device 20 is estimated from the decreased efficiency of the NOx storage-reduction catalyst 30, and the sulfur trap catalyst or the sulfur adsorbent is replaced accordingly. In the related art, however, a minute amount of the sulfur component that passes through the sulfur trap device 20 cannot be detected before the efficiency of the NOx storage-reduction catalyst 30 is decreased.
Conventionally, the amount of the sulfur component that flows into the NOx storage-reduction catalyst 30 is estimated based on the reduced performance of the NOx storage-reduction catalyst 30, and means for detecting sulfur poisoning of the NOx storage-reduction catalyst 30 itself is not provided. In this case, sulfur poisoning of the NOx storage-reduction catalyst 30 usually starts on the upstream side of the catalyst, as shown in the graph C of FIG. 5. Thus, before the performance of the entire NOx storage-reduction catalyst 30 is reduced, it is possible to predict deterioration of the NOx storage-reduction catalyst 30 from changes in a physical property (e.g. temperature) of the front-end surface of the NOx storage-reduction catalyst 30. Meanwhile, as shown in graph D of FIG. 5, depending on an operating condition of the internal combustion engine in which a temperature of the exhaust gas is low, in which an amount of the exhaust gas is large, or the like, the entire NOx storage-reduction catalyst 30 may become subjected to sulfur poisoning in a longitudinal direction. Graph B indicates a detectable level of sulfur poisoning. As described above, the portion of the NOx storage-reduction catalyst 30 that is subjected to sulfur poisoning varies in accordance with the operating condition. Therefore, detection of sulfur poisoning by means such as measuring the temperature of a portion of the NOx storage-reduction catalyst 30 is generally invalid because of its inaccuracy.
Japanese Patent Application Publication No. 2000-45753 (JP-A-2000-45753) describes an exhaust purification system for an internal combustion engine that includes: a NOx adsorbent that adsorbs the sulfur component in the exhaust gas; and a SOx sensor that is provided upstream of the NOx adsorbent to determine deterioration in a NOx adsorbing capacity of the NOx adsorbent, which is caused by sulfur poisoning of the NOx adsorbent.
In the exhaust purification system that described in JP-A-2000-45753, the SOx sensor detects the concentration of SOx in the exhaust gas in real time based on the potential difference between electrodes that each includes a SOx reduction catalyst such as platinum. Then, based on a detected value, the SOx sensor calculates an accumulated amount of SOx that is estimated to be adsorbed by the NOx adsorbent.
It is described in JP-A-2000-45753 that the sulfur component of several hundred ppm is contained in fuel. However, the amount of the sulfur component in fuel has been reduced with improved fuel properties, and today, less than ten ppm of the sulfur component is contained in fuel, and thus is extremely lean. Accordingly, it has become difficult to detect the sulfur component in the passing exhaust gas in real time based on the potential difference between the electrodes that each includes the SOx reduction catalyst such as platinum. Therefore, the SOx sensor described in JP-A-2000-45753 is no longer practical.